1. Technical Field
The present invention relates generally to semiconductor technology, and more specifically to siliciding in semiconductor devices to form abrupt junctions by a silicide growth dopant snowplow effect.
2. Background Art
Integrated circuits are made up of hundreds to millions of individual components. One common component is the semiconductor transistor. The most common and important semiconductor technology presently used is silicon-based, and the most preferred silicon-based semiconductor device is a Metal Oxide Semiconductor (“MOS”) transistor.
The transistor contains a gate electrode (usually polysilicon) over a gate dielectric, over a silicon substrate. The silicon substrate on both sides of the polysilicon gate is doped by ion implantation of boron or phosphorus or other impurity atoms into the surface of the silicon substrate, thereby becoming conductive. These doped regions of the silicon substrate are referred to as “shallow source/drain junctions”, which are separated by a channel region beneath the polysilicon gate.
A silicon oxide or silicon nitride spacer, referred to as a “sidewall spacer”, on the sides of the polysilicon gate allows deposition of additional doping to form more heavily doped regions of the shallow source/drain junctions, which are called “deep source/drain junctions”. The shallow and deep source/drain junctions are collectively referred to as “S/D junctions”.
To complete the transistor, a silicon oxide dielectric layer is deposited to cover the gate, the spacer, and the silicon substrate. To provide electrical connections for the transistor, openings are etched in the silicon oxide dielectric layer to the polysilicon gate and the S/D junctions. The openings are filled with metal to form electrical contacts. To complete the integrated circuits, the contacts are connected to additional levels of wiring in additional levels of dielectric material to the outside of the dielectric material.
As transistors have decreased in size, it has been found that the electrical resistance between the metal contacts and the silicon substrate or the polysilicon has increased to the level where it negatively impacts the performance of the transistors. To lower the electrical resistance, a transition material is formed between the metal contacts and the silicon substrate or the polysilicon. The best transition materials have been found to be cobalt silicide (CoSi2) and nickel silicide (NiSi2).
The silicides are formed by first applying a thin layer of the cobalt (Co) or nickel (Ni) on the silicon substrate above the S/D junctions and the polysilicon gates. The semiconductor wafer is subjected to one or more annealing steps at temperatures below 800° C. and this causes the cobalt or nickel to selectively react with the silicon and the polysilicon to form the metal silicide. The process is generally referred to as “siliciding”.
Transistors used in integrated circuits are accordingly made ever smaller as the complexity and packing density of those circuits continue to increase. Those transistors use p-n junctions, which are formed in semiconductor substrates by controlled introduction of one or more of the dopant species in selected areas. Modem, scaled down, high performance devices require these junctions to be shallow and abrupt.
Such junctions, as they are formed by the ion implantation, have ion distribution patterns or profiles in the substrate that are determined by the ion implantation parameters and the substrate properties. Such ion distributions have a finite (i.e., limited) sharpness or abruptness at their edges. The abruptness is then dulled as the dopant undergoes thermal annealing to make it electrically active in the substrate. Such limited abruptness of the dopant profile, and in particular the limited abruptness of the active portion of the dopant profile, poses limitations on the scalability of such devices to very small sizes.
Various methods have been proposed to sharpen the activated dopant profile at the source and drain junctions. These include solid-phase epitaxial regrowth of a preamorphized part of the doped area, as well as shallow and rapid melting of that area by lasers. In both cases, achieved active dopant profiles at the junction can become sharper than the profiles as originally implanted. However, these are complex processes with inherent limitations, and have not fully met the need for better and improved solutions.
Solutions to such problems have been long sought but prior developments have not taught or suggested solutions and, thus, solutions to these problems have long eluded those skilled in the art.